The present invention relates to a material testing system that applies force loads to a test specimen. More particularly, the present invention relates to a test specimen holder that holds the test specimen in the material testing system.
Test specimen holders or grips are well known in the material testing art and are used frequently to hold a test specimen in a material testing system. The holder includes opposed jaws operated by a piston that grips the test specimen therebetween. Preferably, the jaws grip the test specimen without altering the vertical position of the jaws on the test specimen, thereby making it possible to preselect the exact point at which the specimen will be held in order not to impart tension loads during placement of the test specimen in the material testing system.
Fluid pressure (hydraulic or pneumatic) pressurizes the piston chamber to operate the jaws. On some known test specimen holders, operation of the valve can apply unwanted loads to the test specimen. For instance, many test specimen holders utilize a toggle-type switch to control the valve providing fluid pressure to the piston chamber. In some cases, the amount of force required to operate the switch and the direction in which the force is applied may cause unwanted preloading of the test specimen due to small displacements of the test specimen holder.
A testing specimen holder for holding a test specimen in a material testing machine includes a base member having a piston chamber, a valve chamber fluidly coupled to the piston chamber, and a port fluidly coupled to the valve chamber. A piston is moveable relative to the piston chamber. First and second jaw assemblies are supported by the base member so as to be opposite each other. The jaw assemblies are operably coupled to the piston. A valve is disposed in the valve chamber and a valve stem is coupled to the valve. The valve stem is supported by the base member and is twistable to operate the valve. A user operable knob is coupled to the valve stem.